Inspection and metrology technologies are conventionally used in semiconductor wafer facilities for material monitoring, disposition, yield prediction, and yield management. Reticles are inspected at various times to identify changes in a reticle over time. Reticle defects that are changing in time relate to the wafer fabrication requalification where the reticle is inspected to ensure it has not become contaminated during storage or usage. One particularly memory intensive inspection process, called die-to-golden inspection, involves imaging the relevant portions of a known, clean substrate. The clean image is known as a “golden image” of the reticle. The golden image is generally taken immediately after the reticle has been cleaned and verified with a die-to-database inspection. The die-to-database inspection is an optical comparison of the reticle (or portions of the reticle) to a computer rendered image of the design database used to create the reticle.
Once the golden image is stored, subsequent images of the substrate are compared to the golden image. Any variations are identified as changes or contamination to the reticle.
Golden images are on the order of 8 terabytes per reticle. A semiconductor production process may include 1000 reticles. Total storage for golden images for a single semiconductor wafer could exceed 8000 terabytes. 8000 terabytes of golden image data is expensive to store and expensive to utilize. In addition to the cost of maintaining sufficient storage media, golden images may not be stored where the inspection process takes place, so golden images are also expensive in terms of data bandwidth to transmit the golden images to the inspection site. Furthermore, golden images may include images taken of both reflected light and transmitted light; effectively doubling the amount of data.
Data compression can reduce the total amount of storage necessary, but the advantages are limited. A reduction by a factor 5-10 is desirable but cannot be attained through compression alone.
Alternatively, a reticle may be inspected with reference to the semiconductor design database used to produce the reticle. The semiconductor design database is a perfectly accurate reference that may be used to render an image of the resulting reticle. While a semiconductor design database may not consume as much storage space as a series of golden images, the storage requirement is still substantial. A semiconductor design database may be 1 terabyte or more for a single reticle; therefore a semiconductor wafer requiring 1000 reticles still requires 1000 terabytes of storage and corresponding bandwidth to transfer the databases from their storage location to the inspection site. Furthermore, rendering a semiconductor design database is extremely computationally intensive, often requiring a supercomputer.
Another type of inspection process, cell-to-cell inspection, is a mode wherein locally repeating structures are compared to each other, and any noted difference is declared to be a defect. Cell-to-cell inspection has been long used in both wafer and reticle inspection. This modality has advantages in that the reference data is very closely spaced to the test region so that the inspection tool does not need to be particularly stable to successfully employ this approach, and no stored reference image is necessary. Similar to cell-to-cell inspection, die-to-die inspection is a mode of inspection wherein identical dies on the same substrate are compared.
Consequently, it would be advantageous if an apparatus existed that is suitable for performing hybrid inspections to reduce the amount of data necessary during a die-to-golden inspection and die-to-database inspection.